Analysis of samples by using X-rays is well known. The technique of X-ray diffraction (XRD) is used to determine the structural (i.e. crystallographic) properties of a crystalline sample. In XRD, the X-Ray diffractometer usually incorporates a monochromatic X-Ray source, which is typically collimated or focused for irradiating a sample to be analysed and which has an intense characteristic X-ray line, e.g. at 8.043 keV, and a detector which is optimized to detect this radiation after it has been diffracted by the sample. Typically, both the X-ray source and the detector are angularly moved (rotated) around the sample in order to scan the diffraction angle (φ), which angle is shown in FIG. 1. There are many variants of diffraction setups optimized for specific applications. Usually the sample to be analyzed is polycrystalline which means that crystallites of small size are isotropically and randomly distributed in the sample. Depending on their direction, the incident X-rays can reach an ensemble of crystallites which are distributed with a crystal plane oriented in such a way that collective diffraction from these crystallites occurs for a given direction of detection, i.e. along a given diffraction angle (φ) as determined by Bragg's law. There are also issues regarding incident and diffracted beam attenuation along the path length inside the sample which can be mitigated by favoring diffraction from crystallites which are in the vicinity of the surface of the sample. This can be done by tilting the sample. Detection of the diffracted X-rays over a range of angles provides an angular diffraction pattern or diffractogram having characteristic peaks of diffracted intensity when the condition of Bragg's law is satisfied and information on the crystalline structure of the analyzed sample can be obtained from this angular diffraction pattern. However, XRD does not provide information about the elemental composition of the sample.
In order to obtain information on the elemental composition of a sample, different techniques can be used, such as chemical (e.g. titration) or spectroscopic (e.g. optical emission), all having their own advantages and drawbacks. The X-ray fluorescence (XRF) phenomenon can be used to perform elemental analysis and to obtain accurate, quantitative composition information on a sample non-destructively. In the XRF technique, a sample is irradiated with an X-Ray beam which induces the emission of secondary X-rays having wavelengths characteristic of the constituents elements of the material. In order to induce X-Ray fluorescence on the broadest variety of materials, a polychromatic X-Ray source is used to access the widest possible range of wavelengths. XRF spectrometers are of the Energy Dispersive (EDXRF) or wavelength dispersive (WDXRF) type. The principal features of a WDXRF apparatus are: a polychromatic, usually divergent X-Ray source, a dispersing means to select the wavelength peaks of interest and an X-ray detector. The dispersing means is usually a flat or curved crystal, having respectively parallel or focusing beam optics. Changing the angle (θ) between the crystal and the detector allows scanning of the wavelength of the emitted secondary X-rays reaching the detector. State of the art XRF apparatus may incorporate several static or simultaneous detection channels each with its own crystal and associated fixed wavelength which detect simultaneously and/or a rotating device to sequentially scan the wavelengths of interest. However, the XRF technique does not provide information about the structure or crystalline phases of the sample.
In view of the above discussion it can be seen that it would be desirable to provide both XRD and XRF techniques in a single instrument to increase analysis capabilities, as well as to reduce both cost and footprint in the laboratory. However, this is complicated because the requirements of the two techniques are very different as described above.
There are several prior art apparatus for performing XRF and XRD with the same instrument. Some prior art solutions have employed two X-ray sources, e.g. as described in U.S. Pat. No. 3,344,274 and WO 2008/107108 A1. Such designs are clearly complex and expensive. Another prior art design is described in EP 183,043, which uses a single X-ray source but requires a complex arrangement of goniometers in order to produce an apparatus capable of both XRF and XRD. The latter design can result in difficulties due to the precision and reproducibility necessary when the geometry of the device is changed.
Another prior art solution is described in U.S. Pat. No. 5,406,608 which uses a single vertically mounted X-ray source and implements multiple XRF channels mounted azimuthally around the X-ray tube. The XRF channels can only be used for XRF measurements. In addition to the XRF channels, there is a separate XRD monochromatic detection arrangement rotating around the sample. The XRD detection arrangement comprises a collimator, a crystal and a detector. This arrangement is mounted on a support which can be moved around the sample by means of an actuator to record the XRD pattern. This detection arrangement is dedicated to XRD measurement and the angle between detector and crystal is fixed so as to be optimised for XRD. However, this design suffers the drawback that the XRD detection is separate from the XRF detection, which requires additional space and multiple components.
In WO 97/25614, an apparatus with two separate detection systems has been proposed (one scanning WDXRF system optimized for XRF, and a fixed monochromatic system for XRD), at the expense of increased cost, size and complexity. In that case it is noted moreover that the diffraction angle is scanned using only primary collimator tilt, i.e. no goniometer, which allows only scanning a very limited portion of the diffraction spectrum. Another apparatus with two separate detection systems has been proposed in WO 97/13142.
There are also known prior art designs with one or a plurality of monochromatic XRD sources used to illuminate the sample for XRD and XRF measurement, e.g. as described in US 2006/088139A1 or U.S. Pat. No. 6,798,863. However, the instruments cannot perform effective state of the art X-Ray fluorescence measurements because they use a specific XRD radiation source which strongly limits the range of compositions that can be measured by fluorescence. In addition, in these cases the XRF detector is of an energy dispersive (EDX) type and XRF performances are accordingly limited as compared to using a wavelength dispersive type. Moreover, in U.S. Pat. No. 6,798,863 the XRD pattern is recorded using a CCD line or strip detector which again limits performance and requires a specific additional component only used for XRD application. Whilst a strip detector allows simultaneous acquisition of the diffraction pattern, numerous drawbacks include not being usable with parallel (collimating) geometry, potential spectral distortion, insufficient energy discrimination, complex data acquisition electronics and not being usable for peak follow-up, as well as usually being of higher cost.
A common detection arrangement performing for both XRF and XRD is not trivial to achieve, since it requires compromises, e.g. regarding the choice of X-ray source, crystal types and of collimator divergence. Also, detecting wavelengths for XRD using detectors commonly used in XRF is unusual since the energy spectrum of the X-Rays is quite different for XRD and XRF. In U.S. Pat. No. 4,263,510 is disclosed an XRD-XRF apparatus, which is again primarily optimized for XRD as is indicated by its point-like source collimator optimised for the type of anode geometry commonly used in X-ray tubes used for XRD (monochromatic X-ray tubes). The X-ray tube is also positioned relatively far from the sample which results in reduced X-ray fluorescence signals, since XRF apparatus should illuminate an area as large as possible on the sample surface with a polychromatic X-Ray source located as close as possible to the sample to get maximum fluorescence intensity. For the X-ray detector, U.S. Pat. No. 4,263,510 refers to the use of an energy dispersive (EDX) or wave-dispersive (WDX) analyser, although the drawings therein strongly imply the need for energy dispersive detection due to the compact nature of the detector illustrated. Moreover, due to the position of the XRD X-ray tube far from the sample, and due to it appearing to be a monochromatic tube, the detector would be required to be of the energy dispersive type for the apparatus to have any practical use as an XRF apparatus, since the X-ray signal would be too low for effective detection by wave-dispersive analysis and certainly not wavelength dispersive analysis using a wavelength scanning device (monochromator) rather than a polychromator. This limits severely the extent and flexibility of XRF measurements which can be made using the apparatus. Thus, in order to record XRF with good sensitivity as well as XRD it has generally been required to use two separate detection systems for XRF and XRD as discussed above.
In view of the above background the present invention has been made.